3472 J . Org. Chem., Vol. 39, No. 24, 1974
Olah, Schilling, Renner, and Kerekes
Radical Reactions. I. Phosphorus Chloride Catalyzed Chlorination of Alkanes, Cycloalkanes, and Arylalkanes George A. Olah,* Peter Schilling,l Rudi Renner,2 and Istvan Kerekes Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 Received August 1, 1974
Phosphorus pentachloride and trichloride are highly effective catalysts for the radical type chlorination of alkanes, cycloalkanes, and arylalkanes. Reactions can be carried out conveniently at or above room temperature and in the dark. The reaction can be performed in chlorinated solvents of low polarity or in hydrocarbon solvents. Polar solvents, such as nitromethane, cause exclusive ionic chlorination to occur. The scope and mechanism of the novel, Lewis acid catalyzed radical chlorination reaction are discussed. Because of their weak Lewis acidity, PCls and PC13 are only infrequently used as Friedel-Crafts catalysts. Piazzi3 found PC15 as a useful catalyst for cyclizations and Tada4 reported its ability to promote the rearrangement of 1chloro-1-nitrosocyclohexane.The PC15-catalyzed chlorination of aromatics tends to promote preferential chlorination of side chains over ring c h l ~ r i n a t i o na, ~result which is not expected for a Lewis acid catalyzed Friedel-Crafts Chlorination. Wyman, et al., reported that PC15 itself serves as a chlorinating agent for alkanes and arylalkanes when the reactions are carried out at 100" and in the presence of benzoyl peroxide. Kimbrough7 investigated the chlorination of several arylalkanes with PC15 a t elevated temperatures. Kooymans studied the chlorination of norbornane with PC15, initiated by uv light or peroxides. Fell and Kungg observed increased selectivity for chlorination of alkanes with molecular chlorine in the presence of phosphorus trichloride. All these reactions were considered to be typical radical chlorinations.1° In the course of our studies of electrophilic substitution reactions and in particular Friedel-Crafts catalyzed chlorinations, we observed repeatedly the irregular behavior of PCls-catalyzed chlorinations particularly the formation of substantial amounts of side chain chlorinated products in arylalkanes. We undertook now a detailed investigation of the phosphorus chloride catalyzed chlorination of alkanes, cycloalkanes, and arylalkanes to establish the scope and utility of these interesting and little studied reactions and also to clarify the mechanism of the reactions and particularly to distinguish conditions for homolytic or heterolytic chlorination.
Results and Discussion Our investigations of the phosphorus chloride catalyzed chlorinations were carried out in the dark and a t or slightly above room temperature, with radical reactions initiated by light or common radical initiators excluded. Liquid alkanes or arylalkanes were chlorinated either with no solvent used or with hydrocarbons dissolved in nonpolar solvents, such as CC14 or CHZC12. Arylalkanes are nearly exclusively chlorinated in the side chain. Adding a polar solvent (CH3N02) to the chlorination mixtures in CC14 increases the amount of ring chlorination and by reaching 50% nitromethane, exclusive ring chlorination takes place. As is shown in Table I the PC15-catalyzed chlorinations of alkanes (carried out with chlorine under pressure in the dark a t 25") give good yields of haloalkanes (with the exception of the low yield in the case of methane). Since in blank reactions it was found that no chlorination occurred when PC15 was absent, or when the alkanes were kept in the presence of PC15 in the absence of chlorine under the same conditions, we have to assume that PC15 indeed acts as catalyst for the chlorinations. Phosphorus trichloride gave similar results to the
PCl5-catalyzed reactions. PC13, in the presence of chlorine of course, gives PCIB. Thus in solvents of low polarity the equilibrium Pc15 == Pc13 Cl2 must be assumed, explaining the similar results of chlorination observed using both Pc15 and PC13. As all observed reactions gave similar results using either Pcl5 or PC13 (in the presence of chlorine) we are subsequently discussing these reactions only in case of PC15. In considering the way in which PCl5 would catalyze the chlorination of alkanes, first we needed to establish whether the reaction is radical or ionic in nature. This can be best done by determining the selectivity shown in the reactions and comparing them with the selectivities observed in typical radical chlorinations (for example the light-induced chlorination) and in recently observed cationic chlorinations.ll In order to determine the relative selectivity of different hydrogen atoms of alkanes in the PC15-catalyzed chlorination we used a tenfold excess of alkane over chlorine (with the molar ratio C12:PC15 = 1 O : l ) and analyzed the resulting mixtures of isomers by means of glc. T o avoid any side reactions, the mechanistics studies of chlorination were carried out a t 25" with necessarily longer reaction times, although preparative chlorinations (to be reported separately) are more advantageously carried out a t somewhat elevated temperatures. The results are summarized in Table 11. n-Butane in cc14 as solvent gives a RSpSvalue of 3.5 in good accordance with RSpS = 3.9 obtained for the light-induced chlorination in the gas phase a t 35" by Fredericks and Tedder. l 2 The PCl5-catalyzed chlorination of isobutane gives an RSpt value of 3.4 very similar to RSpt = 3.5 obtained for the photochlorination in CC14 by Hodnett and Juneja13 and R u ~ s e l 1 . lWhen ~ chlorobenzene is used as a solvent the chlorination is much more selective, a trend which is generally observed for chlorinations proceeding uia a radical type mechanism.14-l6 For isobutane and butane the values are RSpt = 7.3 and RSPs = 5.1, respectively. 2,4-Dimethylpentane containing primary, secondary, and tertiary hydrogen atoms gives RS values very similar to those which are found for the photochlorination of the neat alkane at 25" or were reported by Russell17 for the photochlorination in CC14 at 40". In the PC15-catalyzed chlorination values of RSps = 2.2 and RSpt = 2.8 respectively, were obtained. In typical cationic chlorinations of alkanes,ll not only C-H bond substitution (chlorination) but also C-C bond cleaving chlorination (chlorolysis) is observed. No such chlorolysis, however, is observed under radical conditions (only perchlorinated alkanes undergo pyrolytic chlorolysis a t elevated temperatures). Since there is no indication of chlorolysis and the selectivities resemble those of radical chlorination, we can assume that the phosphorus chloride catalyzed chlorinations are indeed of radical nature. Phos-
+
J. Org. Chem., Vol. 39, No. 24, 1974 3473
Chlorination of Alkanes, Cycloalkanes, and Arylalkanes
Table I PCle Catalyzed Chlorination of Alkanes a n d Cycloalkanes i n the D a r k at 26" a Hydrocarbon
Registry no.
Molar ratio
Reaction
C1Z:PClS:alkane
time, hr
Reaction products' (%)
Methyl chloride' (1.0) Dichloromethane (22.0) Chloroform (1.0) 42 Ethyl chloride (21.3) l:o. 1:l 74-84 -0 Ethane 1, 1-Dichloroethane (4. 5) 1,2-Dichloroethane (1.8) Isopropyl chloride (17.4) 20 1:O. 1:O. 8 74-98-6 Propane n-Propyl chloride (12.6) 1-Chlorobutane ( 5 . 4 ) 28 106-97-8 1:o. 1:l Butane 2-Chlorobutane (14.6) 1,2-Dichlorobutane (5.0) 1,S-Dichlorobutane (15.0) tert-Butyl chloride (8.0) 30 l:o. 1:2 75-28-5 2 -Methylpropane (isobutane) 1-Butyl chloride (12. 1) 22 Neopentyl chloride (47. 5) 1:o. 1:2 463-82-1 2 , 2 -Dimethylpropane (neopentane) 1,l-Dichioro-2, 2Dimethylpropane (7.2) Cyclopropyl chloride (5.0) 30 75-19-4 l:o. 1:2 Cyclopropane 1, 1-Dichlorocyclopropane (6.2) 1,3-Dichloropropane (11.8) Cyclohexyl chloride (25.0) 30 110-82-7 l:o. 1:2 Cyclohexane exo-2-Chloronorborane (12.0) 60 1:O. l:O. 15 279 - 23 -2 Norbornaned endo-2-Chloronorborane (5.6) 7-Chloronorbornane (10.4) 1-Chloroadamantane (40.0) 1:O. l:O. 05 46 281 -23-2 Adamantanee 2-Chloroadamantane (14.4) 17 1-Chloroadamantane (24.0) l:O. 1:O. 05 Adamantane' 2-Chloroadamantane (16.0) a Without solvent. b Product analysis by glc or nmr spectroscopy, yield of the identifiable products based on the amount of alkane charged. c Analyzed by MS. d Reaction temperature 65". e In CC14 a t 60". f In CHzClz at 25". Methane Methyl chloride
74-82-8 74-87- 3
phorus chlorides also are excellent and highly selective catalysts for side-chain chlorination of arylalkanes. At 25' and in the dark they are capable of effecting almost exclusive side chlorination. For preparative purposes (to be reported separately) again somewhat elevated temperatures (5060°), giving much faster reaction rates, are preferred. Comparing the relative rates of a chlorination of toluene, ethylbenzene, and cumene shows that the change of the a hydrogen from a primary in toluene to a secondary in ethylbenzene increases its reactivity 3.3 times ( k E B / k T = 2.2), whereas the change from a secondary hydrogen in ethylbenzene to a tertiary in cumene increases its reactivity 4.5 times (thus to a higher degree). The values obtained in competitive chlorination of the neat aromatic hydrocarbons show some differences from the results of photochlorination of the same substrates a t 40' reported by Russell.16 The values reported by Russell for the chlorination in nitrobenzene, cyclohexane, or CC14 show relative reactivities of the different C-H bonds toward chlorine atoms and of to1uene:ethylbenzene (a):cumene ( a ) = 1:3.9:8.5 independent of the solvent used. Considering that our results were obtained in chlorination of near substrates, the relative reactivities of to1uene:ethylbenzene (a):cumene ( a ) = 1: 3.3:14.7 seem to indicate that the use of PC15 as catalyst gives comparable selectivities with previous chlorinations. The explanation for some differences could be the increasing stability of the T complexes formed between PC15 and the arylalkyl hydrocarbons going from toluene to cumene, which could favor chlorine attack on the tertiary C-H bond of cumene more than expected on a simple statistical basis. On the other hand the reaction in neat aromatics should also result in the involvement of a more complexed chlori-
1:o. 1:2 l:O. 1:O. 8
22 17
nating agent which is more selective, as was shown by Wibergls and Walling.lg In a study of the kinetic isotope effect for chlorination of t o l u e n e d l k ~ :Dh= 1.3 was found in CC14 as solvent, whereas for the undiluted aromatic hydrocarbon k H/k D is 2.1.18,20 The relative amounts of a and fl isomers formed in the PCl5-catalyzed chlorination of ethylbenzene and cumene are 10.95:l and 8.66:1, respectively, using no solvent. When the substrates are diluted with CCl, the chlorination becomes less selective with increasing dilution (see Table 111).The same trend was found by Russell when he investigated the photochlorination of the same aromatics in nitrobenzene solvent.l6 Comparing the reactivity of the hydrogen atoms in the side chain of toluene with those of cyclohexane, usually used as a standard in the determinatiod of substrate selectivities, the PCls-promoted chlorination gives a relative rate of k cyclohexane/ktoluene = 13.6 in the competitive chlorination which was carried out with a 1:1 mixture of the substrates, without using any solvent. From this result it can be seen that the hydrogen atoms in cyclohexane are 3.4 times more reactive than the hydrogen atoms in the side chain of toluene. Russell and Brown15 reported a relative rate of 11.2 in favor of cyclohexane for the competitive photochlorination at 80°, very close to our result. However, when the results of the competitive chlorination of cyclohexane and toluene are compared with the rate ratio of chlorination of the neat substrates, one finds that cyclohexane reacts much slower than toluene (in 30 hr only 25% cyclohexyl chloride is formed, whereas 51% benzyl chloride is formed in 30 min). In order to clarify this seeming discrepancy we determined the rate of chlorination of cyclohexane in different aromatic solvents (1:l mixtures of cyclohexane and aromatics were
3474 J . Org. Chem., Vol. 39, No. 24, 1974
Olah, Schilling, Renner, and Kerekes
Table I1 Substrate and Positional Selectivity of the PCl5-Catalyzed Chlorination of Alkanes and Arylalkanes in the Dark at 25" a Hydrocarbon
Butane
2-Methylpropane (isobutane)
2,4 -Dimethylpentane
Solvent
Reaction products ( W )
CCl,(PCl,) C,H5C1(PC15) Gas phase 35" CC1, (PC1j) CGHjC1(PC15) CCl,, hv, 24" Neat, kv, -15" C,H,Cl, hu, -15" Neat (PC1,)
1- + 2-Chlorobutane 1- -1 2 -C hlorobutane 1- + 2-Chlorobutane 1- f 2-Chloro-2-methylpropane 1- + 2 -C hloro- 2-methylpropane 1- + 2-Chloro-2-methylpropane 1- + 2-Chloro-2-methylpropane 1- + 2-Chloro-2-methylpropane 1- Chloro -2,4 -dimet hy lpentane (54.8) 2-C hloro - 2,4-dimethylpentane (25.3) 3-Chloro-2,4-dimethylpentane (20.0) 1-C hloro- 2,4-dimethylpentane (52.0) 2-Chloro-2,4-dimethylpentane (25.5) 3-C hloro-2,4-dimethylpentane (22.5) 1-C hloro- 2,4-dimethylpentane (55.1) 2-C hloro-2,4-dimethylpentane (22.9) 3-C hloro - 2,4 -dimet hylpentane (22.0) Cyclohexyl chloride Benzyl chloride Cyclohexyl chloride Benzyl chloride Benzyl chloride a-Chloroethylbenzene (91.6) p-Chloroethylbenzene (8.4) Benzyl chloride a-Chlorocumene (89.6) 8-Chlorocumene (10.4)
Neat (PC1,)
Neat, 1zv
CCl,, hv, 40"
Cyc1ohexane:toluene (1: 1)
Neat (PC1,) hu, 80"
Ethylbenzene :toluene (1:1)
Neat (PC1,)
Cumene:toluene (1:I)
Neat (PC1,)
Selectivity
~
_
RS,S = 3.5 RS,S = 5.1 Rs,S = 3.913 RS,' = 3.4 RSDt= 7. 3 RS,' = 3. 5" RSDt= 4. 516 RSPt = 29j6 RSPt = 2.8
RS,9 = 2 . 2
RS,9 = 2.6
RS,'
2.9
RS,t = 2. 5
RS,S = 2.4" k c / k T b = 13,6
kclk, = 11.2
W k T @
= 4.9c
k c / k c B = 8.66 01
Analysis by glc after 10% of the hydrocarbons were chlorinated. * Represents overall cyclohexane:toluene rate ratio (substrate selectivity) without statistical correction for positions. Relative rates of a-chlorinationwere obtained by nmr spectroscopy; relative selectivities a/P were obtained by glc analysis of the chlorination products of neat ethylbenzene and cumene. a
Table I11 PCls-Catalyzed Chlorination of Alkylaromatics in the Dark at 25" a ~~
~
Hydrocarbon
Toluene (T)
Registv no.
108-88-3
Solvent
% side-chain chlorination
96 ring chlorination
Neat 97.3 2.7 (o/'p = 1.65) CCl, (T:CCl, = l : l O ) b > 99 Traces C,H,N02 (T:NB = 1 : 5 ) b 0 100 (O/P = 1.36) 0 100 (o/p = 1.32) CHSNO2 (T:NM = 1:5)b Neat 10'$7b~-DNB 90 10 Neat + 20% l"ii -DNB 2 98 Ethylbenzene (E) 100-41-4 Neat 98a/P = 10.95 2 CC1, (E:CCl, = 1:5)* 1 0 0 a / ~= 7.24 0 CC1, (E:CCl, = l : l O ) b l O O ~ r / p = 5.08 0 Cumene (C) 98-82-8 Neat 98a/P = 8.66 2 CC1, (C:CC1, = 1:5)b lOOa/p = 5.21 0 CCl, (C:CCl, = 1:10)* l O O ~ r / p = 4.27 0 o-Xylene 95-47-6 Neat 93.4 6.6 108-38-3 Neat 35 65 TII -Xylened 106-42-3 Neat 95 5 p-Xylene 108-67-8 Neat 0 100 Mesitylened a Reactions were carried out with a 10: 1 mol ratio of aromatics:PC15. Chlorine gas was continuously introduced into the solutions. Chlorination products were analyzed by glc. b Molar ratios. c rn-Dinitrobenzene.d Ring chlorination occurs also without PCb.
_
J . Org. Chem., Vol. 39, No. 24, 1974 3475
Chlorination of Alkanes, Cycloalkanes, and Arylalkanes
0 neat cyclohexane 0 toluene 0 benzyl chloride A beneal chloride
A cyclohexane
- toluene 1:l
0 cyclohexane
- ethylbenzene 1:l
CI cyclohexane
-
c m e n e 1:l
A cyclohexane-benzene 111 p cyclohexane-t-butylbenzene 111
A
10
20
30
40
50
70
60
60
90 (min)
reaction time
Figure 1. PC15 catalyzed chlorination of toluene in the dark at 25O
0
1
2
3
4
Reaction time ( h o u r s )
Figure 3. Chlorination of cyclohexane. Influence of aromatic solvents on the reaction rate (ratio cyclohexane: PC15 = 10: 1).
4
toluene
0 ethylbenzene
A cumene
100% cc1;
0
50 50
Figure 2. Dependence of the substitution in the PCls catalyzed chlorination of arylalkanes with varying amount of apolar (cc14) and polar (CHsN02)solvent. used). Data showed that the amount of cyclohexyl chloride is substantially enhanced when an alkylated aromatic is used as solvent. Benzene itself gives an increase of the rate of chlorination which is substantially further enhanced when using (in sequence) tert- butylbenzene, toluene, ethylbenzene, and cumene. During the chlorination the amount of added alkylaromatics remains nearly constant at about 80% (with 20% dichlorinated product formed) of the introduced amount, whereas cyclohexane is continuously used u p as its chlorination proceeds. The observation that cyclohexane is more reactive in competitive experiments than toluene, but reacts more slowly in individual noncompetitive runs, may be, however, misleading.21 The rate of a radical chain reaction depends on the composite rate constant, including termination as well propagation rate constants, whereas competitive ex-
periments only evaluate the latter. In the absence of independent information concerning the termination reaction comparison between the noncompetitive experiments may be invalid, as the propagation rate in neat cyclohexane may be slow. Till further data are obtained, we are consequently not attempting any conclusions. In addition to the selectivity studies we also investigated the time dependence of the chlorination by introducing a constant stream of chlorine through a mixture of toluene and PC16 (1O:l) in the dark, at 2 5 O , and determining the reaction products by nmr and glc. As is seen from Figure 1 the chlorination of toluene is very selective and benzyl chloride free from benzal chloride is formed up to about 50% conversion, whereafter further chlorination increasingly gives benzal chloride. The use of polar solvents changes the course of the PC16-catalyzed chlorination drastically, as was shown in the case of toluene. In nitrobenzene or nitromethane solution only ring-substituted isomers were obtained. Since Russell16 found in the case of the photochlorination of toluene in nitrobenzene as solvent no such change and alkylbenzenes gave only side-chain chlorinated products, the change from radical-type side-chain chlorination in CC14 to ionic-type ring chlorination in nitrobenzene or nitromethane solutions in the presence of PC15 must be due to a basic change in the catalytic behavior of this weak Lewis acid when changing the solvent polarity. Figure 2 shows the result of chlorination of toluene, ethylbenzene, and cumene, compounds whose reactivities toward atomic chlorine differ only slightly in dependence of the solvent polarity by changing the relative amounts of CC14 and nitromethane in this mixed solvent system. A continuous change from radical to ionic chlorination is observed. With an increasing dielectric constant of the solvent system and a ratio of CC14: CH3N02 = 40:60 v/v or less, only ring chlorination is observed. We also studied the relative rate of the PCls-catalyzed chlorination of benzene and toluene in nitromethane using the competitive method. The obtained values for the substrate selectivity k T / k B = 62.5 (25') and the isomer distribution o:m:p = 56.9:0.1:43.0%show that PC16 is a relatively weak Friedel-Crafts catalyst for electrophilic aromatic ring
3476 J. Org. Chem., Vol. 39, No. 24, 1974
Olah, Schilling, Renner, and Kerekes Table IV Chlorination of Arylalkanes in the D a r k at 25" a
PCl&atalyzed
Reaction
-
Hydrocarbon
Toluene
Solvent
Neat Nitromethane
Ethylbenzene
Neat Nitroinethane
Cumene
Neat Nitromethane
ievl-Butylbenzene
Neat Nitromethane
o-Xylene
Neat Nitromethane
nz-Xylene
Neat Nitromethane
a
p-Xylene
Neat
Mesitylene
Neat
Chlorine was continuously introduced into the system.
time, min
Reaction products ( 9 6 )
Benzyl chloride (51.0) Chlorotoluenes (1.0) 30 o-Chlorotoluene (45.5) Yn-Chlorotoluene (
